This invention relates to the manufacturing of abrading tools, and particularly to forming the abrading surfaces of lapping plates used in high precision lapping of magnetic transducing heads.
Magnetic transducing heads, used to store and retrieve data on rotatable magnetic recording discs, call for fine manufacturing tolerances, often measured in microinches (millionths of an inch). Thin film heads typically are formed by applying layers of an electrically conductive material and a magnetic flux conducting core or pole-piece material along one side of a comparatively large body or slider. In use, a finely machined planar bottom surface of the slider is spaced vertically apart from a horizontal magnetic recording surface of the rotating magnetic disc, supported by a thin film of air. To form the transducer bottom surface, high precision abrading equipment is used, including a rotating lapping plate having a horizontal lapping surface in which abrasive particles such as diamond fragments are embedded. An abrasive slurry, for example a water soluble glycol base containing diamond fragments or other abrasive particles, is applied to the lapping surface as the lapping plate is rotated relative to the slider or sliders maintained against the lapping surface. The diamond fragments can be from one to two hundred and fifty microinches in diameter. An example of such lapping plate, along with a carrier arm for maintaining a slider bar or other workpiece against the lapping plate, is disclosed in U.S. Pat. No. 4,536,992 (Hennenfent et al).
Common practice is to periodically refurbish the plates with a lapping grit, to produce a surface texture suitable for the embedding and retention of the appropriate size of diamond grit being used with the lapping process. A problem with this is that the surface is susceptible to a rapid change in smoothness as it is used to lap a workpiece, principally due to fragments removed from the workpiece during lapping. The change in smoothness affects the hydrodynamic bearing film provided by the liquid component of the abrasive slurry, creating a "hydroplaning" effect which raises the workpiece from the lapping surface, to diminish the abrasive action of the particles and substantially increase abrading time.
The general idea of interrupting the lapping surface, for example by forming grooves in a lapping plate, is known in the art. For example, U.S. Pat. No. 3,921,342 (Day) shows a lapping plate 12 in which a plurality of troughs are formed in the lapping surface. A filler of material can be placed in the troughs, so that unspent abrasive liquid is maintained adjacent the working surface of the lapping plate, while spent abrasive fluid is centrifugally removed beyond the lap plate periphery. In U.S. Pat. No. 4,037,367 (Kruse), grooves are formed between working surface areas in which an abrasive such as diamond particles are embedded in a metallic coat. The grooves sweep beneath the workpiece to remove abrasive particles as the abrasive disc rotates. Kruse teaches the depth of the groove should be at least twice the nominal diameter of the particles, and the groove width should be at least ten times the nominal diameter. U.S. Pat. No. 3,683,562 (Day) also discloses a grooved lapping plate.
A problem with grooved plates, however, is due to excessive width and depth of grooves. Abrasive particles entering excessively deep grooves are in effect lost, as they become too far removed from the workpiece surface to provide any further abrasive action. This removal of the grit may be caused by steep, nearly vertical side walls of the grooves, as well as the groove depth. Further, the wide grooves provide a surface discontinuity too severe for small workpieces. Forming such grooves is costly and time consuming. Even if the grooves can be sized properly, substantial segments of the lapping surface remain ungrooved, or alternatively a prohibitively large number of grooves are required. Surface uniformity--on the microscopic scale suitable for lapping small workpieces--could be achieved only with extreme care. Refurbishment of such a lapping surface would require renewal of the grooves as well, further adding to the expense.
Therefore it is an object of the present invention to provide a lapping tool having a selected texture for discontinuity over its lapping surface, and on a microscopic scale appropriate for lapping small workpieces.
Another object is to provide a textured lapping surface which is substantially uniform.
Another object is to provide a process for forming, in a lapping tool, a substantially uniform textured lapping surface, while avoiding the expense of cutting grooves in the lapping plate.
Yet another object of the invention is to provide a lapping tool having a uniformly textured lapping surface amenable to repeated refurbishment by conventional processes.